What is CRON and What Can it Do? - Cron without training wheels
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For those of you without a nice GUI to configure your cron, you will need to go inside the crontab directly to configure it. The file is located at /etc/crontab. When Linux is installed, your crontab might look a little like this:
The above file excerpt is how Redhat’s cron looks after installation. It is separated into two parts.
The first part, at the top, sets some variables for the cron when it runs. It sets which shell to use, which folders to search, and a couple other things. These are self explanatory. The difficulty comes when you get to the bottom part. See all of those numbers and asterisks(*)? Those are used to configure when a cron is run, who it runs as, and what it executes.
In the above example, the first scheduled job runs at one minute after the hour, every hour. How did I know that? Let's take a quick look at how the file is configured. The columns are, in this order:
1) Minute (0-59)
2) Hour (0-23)
3) Day of month (1-31)
4) Month (1-12)
5) Day of week (0-7 0 and 7 are BOTH Sunday)
6) User to run cron as
7) Command to execute when the cron runs
For option you don’t want to specify a value, * is used as a placeholder.
In the above example, the third scheduled task runs at 4:22am, every Sunday. It executes the command “run-parts /etc/cron.weekly” as root.
Simple right?
One last thing I’d like to touch on is the "run-parts" option you see in the included example. This script causes all files within a folder to be executed. In the above example, the cron would execute at 4:22 AM on Sunday, and every executable file inside /home/cron.weekly would be executed. This makes it easy to schedule jobs at regular intervals without the need to modify the crontab directly each time.
Final Thoughts
As I said earlier, cron is only available on Linux or other Unix variants. Windows has something called the task scheduler which I will not be getting into.
As you have seen during this introduction, cron can add both complexity and simplicity to your system at the same time. The ability to schedule mundane tasks makes it easy for one person to feasibly run and maintain multiple servers and websites.
In this article, I talked about what cron is used for, and gave some real-life examples where cron has been utilized to enhance the functionality of a site, while at the same time freeing up the server admin from most of the mundane tasks they do on a repeated basis.
I hope I’ve been able to give you enough of an understanding about cron to enable you to decide whether you need to use it.
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